The pulmonary artery, which connects the heart to the lungs, has an inner radius of 2.8 mm and is 8.5 cm long. If the pressure drop between the heart and lungs is 400 Pa, what is the average speed of blood in the pulmonary artery? I am using the equation pi*r^4(p1-p2)/(8*viscosity*length) Would the p1-p2 be -400N or 400N? I thought a speed couldn't be negative... but it says "pressure drop". Would the answer be in m/s? I'm lost.